Insulin has previously been shown to increase the binding of insulin-like growth factor-II (IGF-II) to rat adipose cells. Detailed isotherms of IGF-II binding have indicated that insulin increases the affinity (Ka) of the IGF-II receptor and not the number of receptors. These studies did not take membrane cycling into consideration. The mechanism of insulin-stimulated IGF-II binding has, therefore, been examined by measuring IGF-II binding to intact cells, and plasma and low-density microsomal membranes, in the absence and presence of 1.0 mM KCN to inhibit energy-dependent membrane cycling. In the absence of 1.0 mM KCN, insulin appears to increase the Ka of the IGF-II receptor in the intact cell by 2-3-fold while the number of receptors remains unchanged. However, addition of KCN after preincubation results in an increase in the number of receptors by insulin from 0.040 to 0.179 amol/cell without influencing the Ka (0.2 nN to the -1). In contrast, when cells are fixed with KCN prior to incubation with insulin, no effects of insulin on the number or the Ka or the IGF-II receptor are observed. In addition, insulin increases the number of receptors in plasma membranes by 50% and correspondingly decreases the number in the microsomes, without influencing the Ka in either fraction (0.2 nM to the -1), regardless of the presence of KCN after incubation with or without insulin. These results suggest that 1) IGF-II receptors in rat adipose cells rapidly cycle between the plasma membrane and a low-density microsomal pool, 2) insulin stimulates IGF-II binding through a redistribution of receptors of constant Ka from this pool to the plasma membrane, and 30) caution must be observed when binding is measured to receptors of intact cells undergoing rapid cycling since Scatchard analysis is no longer valid in determining binding parameters.